Aluminum casting method with helium insertion

ABSTRACT

Helium gas at ambient pressure is inserted into an expendable mold at a fill tube or other port exiting the bottom of the mold. The helium displaces atmospheric air in the mold cavity, and the displaced air exits the cavity through the fill tube. When the air in the mold cavity and fill tube has been completely displaced by the helium, the fill tube is submerged in molten aluminum, and the aluminum is drawn into the mold cavity through the fill tube to form a casting having significantly reduced oxide inclusions and significantly improved fatigue strength.

TECHNICAL FIELD

The present invention relates to aluminium casting processes, and more particularly to a casting method in which atmospheric air in a mold cavity is replaced with helium gas to minimize oxide formation in the casting.

BACKGROUND OF THE INVENTION

Various casting processes have been utilized to produce high-strength aluminum parts. Intricate, high precision parts such turbocharger impeller wheels are typically produced using an expendable mold process where molten aluminum is poured or drawn into a plaster or sand mold which is then destroyed to release the casting. Although high quality impeller wheels may be produced very cost effectively in this way, care must be taken to limit exposure of the molten aluminum to atmospheric air in order to minimize the formation of aluminum-oxide in the casting. Oxide inclusions in a casting impair fatigue strength and are believed to be responsible for the majority of fatigue failures that occur in the field.

One way of limiting exposure of the molten aluminum to air is to use suction or pressure to draw the aluminum into the mold cavity through a fill tube that is partially submerged in the molten aluminum. However, this technique is not totally satisfactory due to air resident in the mold cavity and fill tube.

A relatively complicated but more effective way of limiting exposure of the molten aluminum to air is to replace air in the mold cavity and fill tube with an inert gas such as argon or nitrogen prior to filling the mold with aluminum. See, for example, the U.S. Pat. No. 4,027,719 to Strempel, where the technique is applied to a centrifugal casting process, and the U.S. Pat. Nos. 3,900,064; 4,791,977; and 5,042,561 to Chandley et al., where the technique is applied to a counter-gravity or vacuum-assist casting process. However, these known techniques require sealed chambers and various other special equipment that make them both difficult to implement and costly to practice. Accordingly, what is needed is an improved casting method that achieves the low-oxide advantages of known inert gas casting methods, but in an easily implemented and more cost effective way.

SUMMARY OF THE INVENTION

The present invention is directed to an improved aluminum casting method in which atmospheric air in the cavity of an expendable mold is replaced with helium gas prior to introducing aluminium into the mold cavity. The mold is sealed at its top if gas permeable, and helium gas at ambient pressure is inserted into the mold cavity through a fill tube or other port exiting the bottom of the mold. The helium naturally rises into the mold cavity and drives atmospheric air out of the mold cavity and through the fill tube. When the air in the mold cavity and fill tube has been completely displaced by the helium, the fill tube is submerged in molten aluminum, and the aluminum is drawn into the mold cavity through the fill tube to form a casting having significantly reduced oxide inclusions and significantly improved fatigue strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates helium insertion through the fill tube of a plaster mold according to the present invention;

FIG. 1B illustrates helium insertion through a sealed port of a plaster mold according to the present invention;

FIG. 1C illustrates helium insertion through a fill tube of a sand mold according to the present invention;

FIGS. 2A and 2B illustrate the method of the present invention as applied to a counter-gravity or vacuum-assist casting process. FIG. 2A depicts a helium insertion step, and FIG. 2B depicts a casting step.

FIGS. 3A and 3B illustrate the helium insertion method of the present invention as applied to a low pressure casting process. FIG. 3A depicts the helium insertion step, and FIG. 3B depicts the casting step.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is disclosed herein primarily in respect to casting an aluminum turbocharger impeller wheel with an expendable mold. Typically, the mold is formed of permeable plaster, and molten aluminum is drawn into the mold cavity by applying suction to the exterior periphery of mold. However, it should be understood that the method may be applied to other molding processes and to other products, a few of which are briefly discussed herein.

In general, the method of the present invention involves replacing atmospheric air in a top-sealed mold cavity with helium gas prior to introducing aluminium into the mold cavity. In addition to being inert, helium gas is lighter than air. When the helium gas is inserted through the fill tube or an opening in the bottom of the mold, it naturally rises into the mold cavity, expelling the atmospheric air that formerly occupied the mold cavity. FIGS. 1A and 1B illustrate the process as applied to a permeable plaster mold and FIG. 1C illustrates the process as applied to a sand mold.

Referring to FIG. 1A, the reference numeral 10 designates a permeable plaster mold comprising a body 10 a, a cavity 10 b and a fill port 10 c that extends through the body 10 a and into the cavity 10 b. The mold 10 is oriented so that the fill port 10 c opens downward as shown, although the fill port 10 c could extend laterally through the lower portion of the mold body 10 a if desired. A non-permeable cover 12 prevents lightweight gases in the mold cavity 10 b from escaping upward through the permeable mold body 10 a. Before introducing molten aluminum into the cavity 10 b, a conduit 14 is positioned as shown so that a nozzle 14 a at its tip is disposed under or in the fill port 10 c. Helium gas delivered through the nozzle 14 a enters the mold cavity 10 b as indicated by the upward pointing arrows 16. Since the helium gas is lighter than air, it rises and displaces the air in the top of the cavity 10 b; the displaced air exits the cavity 10 b through the fill port 10 c as indicated by the downward pointing arrows 18. Eventually, the mold cavity 10 b and fill port 10 c completely fill with helium gas, displacing all the air that formerly occupied that volume, provided that no part of the mold cavity 10 b lies below the top of the fill port 10 c. The cover 12 prevents the helium from escaping upward through the permeable mold body 10 a. When the mold cavity 10 b and fill port 10 c are completely filled with helium gas, the conduit 14 is removed and molten aluminum is drawn into the mold cavity 10 b via the fill port 10 c to form a substantially oxide-free cast aluminum part.

FIG. 1B illustrates an alternate embodiment involving an expendable mold 20 having a permeable plaster body 20 a, a cavity 20 b, a fill port 20 c and a separate helium insertion port 20 d. In other respects, the embodiment of FIG. 1B is like that of FIG. 1A, and the same reference numerals have been used to designate corresponding elements in both figures. In the embodiment of FIG. 1B, the helium conduit 14 is positioned so that its nozzle 14 a seats on the exterior periphery of the mold body 20 a surrounding the helium insertion port 20 d. Helium gas thus enters the mold cavity 20 b through the helium insertion port 20 d as indicated by the upward pointing arrows 16. Air in the cavity 20 b that has been displaced by the helium gas is expelled through the fill port 20 c as indicated by the downward pointing arrows 18. After the helium conduit 14 is removed, the helium insertion port 20 d may be plugged before proceeding to the casting phase of the process.

FIG. 1C illustrates yet another embodiment involving an expendable mold 30. In this case, the mold 30 comprises a body 30 a of non-permeable compressed sand, a cavity 30 b, a fill port 30 c and a purge passage 30 d for purging gases in the cavity 30 b during casting. The purge passage 30 d is effectively an extension of the mold cavity 10 b, extending downward and then upward in a U-shaped course before exiting the mold body 30 a at purge port 30 e. As in the embodiment of FIG. 1A, nozzle 14 a of conduit 14 is positioned under or in the fill port 30 c, and helium gas delivered through nozzle 14 a enters the mold cavity 30 b as indicated by the upward pointing arrows 16. Likewise, air displaced by the helium gas is expelled through the fill port 30 c as indicated by the downward pointing arrows 18. The helium gas fills the upper portion of purge passage 30 d, but cannot escape through the purge port 30 e due to the downward excursion of the purge passage 30 d. When the mold cavity 30 b and fill port 30 c are completely filled with helium, molten aluminum is drawn or pumped into the cavity 30 b through the fill port 30 c, and the displaced helium gas is expelled into the purge passage 30 d and out the purge port 30 e.

FIGS. 2A-2B and 3A-3B illustrate the helium insertion method as applied to two different representative casting processes. FIGS. 2A-2B illustrate a counter-gravity or vacuum-assist casting process, whereas FIGS. 3A-3B illustrate a low-pressure casting process.

Referring to FIGS. 2A-2B, the reference numeral 50 generally designates an assembly including a permeable plaster mold 52. The mold 52 is surrounded by non-permeable elements, including a top plate 54, an annular flask 56 and a bottom plate 58 with integral fill tube 60. The mold 52 includes a cavity 52 a and a fill port 52 b that aligns with a fill passage 62 within fill tube 60. An annular recess 64 formed on the inner periphery of flask 56 is coupled to suction tube 66 for the purpose of drawing molten aluminum into the cavity 52 a during the casting step of the method. The assembly 50 is adjustably supported over a crucible 68 of molten aluminum 70, topped by an apertured cover plate 72.

Prior to casting, the assembly 50 is raised above the crucible 68 as illustrated in FIG. 2A, and a helium conduit 74 is positioned so that its nozzle 74 a is disposed just inside the fill passage 62 of fill tube 60. The helium conduit 74 is coupled to a helium gas reservoir 76 through a conventional valve (V) 78 that is manually opened to release helium gas through the nozzle 74 a. During the helium insertion process, no suction is applied to the suction line 66, and the helium progressively displaces the air in mold cavity 52 a substantially as described above in respect to FIG. 1A. When the cavity 52 a and fill passage 62 are completely filled with helium, the assembly 50 is lowered onto the cover plate 72 as shown in FIG. 2B. The fill tube 60 of the assembly 50 extends through the aperture 72 a of cover plate 72 and into the molten aluminum 70 as shown. Negative pressure is then applied to the suction tube 66 to draw molten aluminum into the mold cavity 52 a, and the assembly 50 is raised to release the mold 52 once the aluminum has cooled sufficiently.

Referring to FIGS. 3A-3B, the reference numeral 80 generally designates a movable upper assembly including a permeable plaster mold 82 surrounded on its top and sides by a non-permeable upper flask 84. The assembly 80 is adjustably positioned over a stationary lower assembly 86 including a crucible 88 filled with molten aluminum 90, a covered crucible housing 92, and a lower flask 94 with integral fill tube 96. The lower flask 94 rests on the crucible housing 92, and the fill tube 96 passes through an opening 92 a in the crucible housing 92 and into the molten aluminum 90. Movement of the upper assembly 80 is effected by a support structure including the stationary vertical support arms 98, 100 and the movable support arms 102, 104, 106. The movable support arm 102 rides in tracks (not shown) of the stationary support arms 98, 100, and the movable support arms 104, 106 couple the support arm 102 to the upper flask 84 of assembly 80. FIG. 3A illustrates a raised position of the assembly 80 where the upper flask 84 is vertically separated from the lower flask 94, while FIG. 3B illustrates a lowered position where the upper flask 84 is sealed against the lower flask 94.

Prior to casting, the upper assembly 80 is moved to the raised position depicted in FIG. 3A, and the helium conduit 74 is positioned so that its nozzle 74 a is disposed just inside the fill port 82 b of mold 82. The helium conduit 74 is coupled to helium gas reservoir 76 through valve (V) 78 as described above in reference to FIG. 2A. As helium gas is dispensed through the nozzle 74 a, it progressively displaces the air in mold cavity 82 a as described above in respect to FIG. 1A. When the cavity 82 a and fill port 82 b of mold 82 are completely filled with helium, the upper assembly 80 is lowered onto the stationary lower assembly 86 as illustrated in FIG. 3B. Positive pressure is then applied to a conduit 110 passing through a wall of crucible housing 92. Positive pressure in the housing 92 causes the molten aluminum 90 in crucible 88 to rise up through fill tube 96, filling the cavity 82 a of plaster mold 82. When the aluminum in the mold cavity 82 a has cooled sufficiently, the upper assembly 80 is raised to release the mold 82, completing the casting process.

In summary, the present invention provides an inexpensive and easily implemented method for producing high quality aluminum castings with minimal oxide inclusions and significantly improved fatigue strength. While described with respect to the illustrated embodiments, the method may be readily applied to various casting processes, including gravity-fed processes and processing in which metal is pumped into a mold cavity. Also, it should be recognized that numerous modifications and variations in addition to those mentioned herein will occur to those skilled in the art. Accordingly, it is intended that the invention not be limited to the disclosed embodiment, but that it have the full scope permitted by the language of the following claims. 

1. A method of casting molten aluminum in an expendable mold having a cavity, a body surrounding said cavity and a fill passage that extends through said body and into said cavity, the method comprising the steps of: orienting said mold so that said fill passage opens in a downward direction; dispensing helium gas at ambient pressure in or under said fill passage so that the helium gas naturally rises through said fill passage and into said cavity to displace atmospheric air in said cavity and said fill passage by virtue of the helium's light weight relative to said atmospheric air; and filling said cavity with molten aluminum to form a casting.
 2. The method of claim 1, where said body of said expendable mold is formed of permeable material, and the method includes the step of: sealing at least a top surface of the mold body to prevent said helium gas from escaping upward out of said cavity.
 3. The method of claim 1, where the atmospheric air displaced by said helium gas is expelled through said fill passage.
 4. The method of claim 1, including the steps of: positioning a helium dispenser in or under said fill passage; and dispensing helium gas at ambient pressure with said helium dispenser to fill said cavity and fill passage with helium gas.
 5. The method of claim 1, wherein said mold includes a purge passage for purging gases in said cavity when said cavity is filled with molten aluminum, and the method includes the step of: providing a downwardly extending course in said purge passage such that said helium gas cannot escape through said purge passage prior to the filling of said cavity with molten aluminum.
 6. The method of claim 1, wherein said casting is a turbocharger impeller wheel.
 7. The method of claim 1, including the steps of: securing a fill tube to said fill passage; partially submerging said fill tube in said molten aluminum; and filling said cavity with molten aluminum through said fill tube and said fill passage. 